1. Field of the Invention
The present invention relates to a control device enabling an inkjet printer to reliably provide high-quality images at a high printing speed.
2. Related Art
There has been proposed a line scanning typo inkjet printer capable of printing images on an elongated uncut recording sheet at a high printing speed. This type of printer includes a head that is formed with a plurality of nozzles and has an elongated width covering across the entire width of the recording sheet. When printing images, ink droplets are ejected from the nozzles based on recording signals onto the recording sheet that is being fed at a high speed in its longitudinal direction. By controlling both the ink ejection and the feed of the recording sheet, a desired image is obtained on the recording sheet.
There are two types of line scanning type inkjet printer. One includes a continuous inkjet head, and the other includes an on-demand inkjet head. Although the printer with the on-demand inkjet head is slow in printing speed compared to the printer with the continuous inkjet head, the on-demand inkjet head requires a simple ink system, and so is well suited for general-purpose high-speed printers.
An on-demand inkjet head of a line-scanning type inkjet printer is formed with a plurality of nozzle lines, each including a plurality of nozzles aligned in a line. Each of the nozzles is formed with an ink chamber and provided with an energy generating member, such as a piezoelectric element or a heat generating element. Upon applied with a driving voltage, the energy generating member applies a positive pressure to ink in the ink chamber, so that some of the ink is ejected as an ink droplet through a nozzle hole.
There has been proposed an inkjet printer that includes the above-described on-demand inkjet head and, in addition, charger/deflector mechanism, which charges an ink droplet ejected from the nozzle and also generates a deflector electric field that deflects the charged ink droplet in flight so that the deflected ink droplet will alight (impact) a desired position on the recording sheet In this type of inkjet printer, a plurality of ink droplets ejected from different nozzles can be controlled to alight the same single spot on the recording sheet in order to form a single dot thereon. Because each dot on the recording sheet is formed from a plurality of ink droplets from different nozzles, even if one or more of the different nozzles become defective, the dot is still formed by the reining nozzle(s), whereby images can be formed reliably Also, because each dot is formed by a plurality of different nozzles, bands of darker or lighter gray tones and lines on the printed image due to uneven characteristics among the plurality of nozzles can be canceled out, and so a high quality image, without uneven color density or a white line across the page, can be provided.
Japanese Patent Publication (Kokoku) No. SHO-47-7847 also discloses an ink-droplet deflecting theory for deflecting ink droplets using a charging-amount control method. That is, ink droplets ejected from nozzles are charged based on recording signals, and the charged ink droplets fly through an electro-static field, which deflects the charged ink droplets. The deflection amount depends on the charging amount of the ink droplets. Because it is possible to deflect ink droplets ejected even at a high frequency, this method is well suited for a high-speed printing.
However, when ink ejection is performed in a two-droplet mode where an ejected ink droplet is separated into a main ink droplet and a satellite ink droplet during the flight before reaching the recording sheet, deflection amounts of the main ink droplet and the satellite ink droplet will defer, whereby the impact position of the main ink droplet will defer from that of the satellite ink droplet. In this case, a single dot is not properly formed on the recording sheet, but undesirable two separate dots are formed, resulting in degradation in overall image quality.
In order to overcome the above problems, it is necessary to perform ink ejection in a single-droplet mode where the ejected ink droplet is not separated during the flight or even it separated, a main ink droplet and a satellite ink droplet merge into a single ink droplet immediately after the separation. Here, the ink ejection performance will be leas influenced by the environmental factors when the ink ejection speed is set higher. Therefore, it is preferable to set the ejection speed relatively high in order to prevent environmental factors from affecting the ink ejection performance. However, although it is relatively easy to achieve the single-droplet mode with a relatively slow ink ejection speed, when the ejection speed is high, then main and satellite ink droplets will not merge easily, resulting in undesirable two-droplets mode.
In this manner, ink ejection speed affect the droplet mode, i.e, either the single mode or the plural mode, such as the two-droplet mode. In addition, the droplet mode also depends on other factors, such as a nozzle type, an ink type, an ink temperature, and the like For example, when ambient temperature changes, ink properties, such as viscosity and surface tension, also change even when other factors or parameters, such as the nozzle properties and ink type, are unchanged When the ink properties change, then the droplet mode may also change, so that an ink ejection speed range within which the single droplet mode can be achieved may change. For example, even when a device can achieve the single droplet mode at the room temperature, the device may be able to achieve only the two-droplet mode at a higher or lower temperature oven if any other parameters are unchanged. Because the effective ink ejection speed range is limited even with the uniform nozzle properties and a single type of ink, when nozzle properties varies and/or a variety of inks is used, then the effective Ink ejection speed range will be limited even more. In fact, it is difficult to make all the nozzles to have the uniform properties, and various types of inks are used in actual printing. Hence, an operational tolerance level of the device designed for a single-droplet mode only is undesirably limited.
It is an object of the present invention to overcome the above problems and also to provide a control device that realizes a highly-reliable inkjet printer capable of printing high quality images at a high speed with a high operational tolerance level even in the two-droplet mode.
In order to achieve the above and other objects, there is provided a control device used in combination with an ejection unit that ejects an ink droplet toward a recording medium, wherein the ink droplet is divided into a plurality of sub-droplets during flight before reaching the recording medium. The control device includes an electric field generating unit that generates a first electric field that redistributes charge within an ink droplet before the ink droplet is divided into a plurality of sub-ink droplets.
There is also provided an inkjet printer including an ejection unit that ejects an ink droplet toward a recording medium, wherein the ink droplet is divided into a plurality of sub-droplets during flight before reaching the recording medium, and an electric field generating unit that generates a first electric field that redistributes charge within the ink droplet before the ink droplet is divided.
Further, there in provided a control method of controlling impact position of sub-droplets. The control method comprises the steps of a) ejecting an electrically charged ink droplet, b) redistributing charge within the charged ink droplet before the charged ink droplet is divided into a plurality of sub-droplets, and C) deflecting the plurality of sub-droplets.